Voltage controlled lc tank oscillator

ABSTRACT

Voltage controlled oscillator comprising a LC tank circuit (L, C, R) coupled to modulator means ( 2 ) and characterized in that the modulator means ( 2 ) are coupled to amplifier means ( 1 ) via an adder ( 3 ) for generating a quadrature periodical output signal having a frequency in a relative wide range, the frequency being controlled by a control signal (V T ) provided to the modulator means ( 2 ).

The invention relates to a voltage-controlled oscillator comprising a LCtank circuit coupled to modulator means. The invention also relates to aphase locked loop including such an oscillator.

The need for generation of quadrature signals for wireless and opticaltransceivers within a large frequency range comes from the low IF/zeroIF architectures for wireless and architectures for optical receivers.In order to cover different standards with the same oscillator theoscillator should be tunable over a large frequency range. When octavetuning is possible simple frequency dividers based on two D flip-flopscould be used and any frequency below the oscillation frequency of theVCO could be synthesized. This simplifies a lot the divider architectureand reduces power consumption. Usually, the price paid is the need forRC oscillators since they are tunable in a wide range. LC oscillatorshave better phase noise than RC oscillators but are tunable in a smallerrange. The state of the art LC oscillators with large tuning range arebased on two LC tanks with either an interpolation mechanism among thetwo or a ring construction, which could be seen as a combination of a RCand a LC oscillator. The latest construction provides quadrature outputsat the oscillation frequency but they are not tunable in a widefrequency range e.g. at least an octave.

U.S. Pat. No. 6,198,360 discloses a circuit and a method used in LC orring oscillators. A frequency of oscillation of the oscillator may bemodulated by detecting a quadrature signal and controlling the sign andmagnitude of the quadrature signal that may be feedback to theoscillator to cause the oscillator to run either faster or slower(dependent on the sign of the quadratic signal) than the unmodulatedoscillator. The circuit comprises a cross-coupled pair of transistorsfor implementing a negative resistance necessary for frequencygeneration. A LC circuit determines the oscillation frequency. Twocapacitors C1, C2 and two resistors R1, R2 are provided for obtainingquadrature oscillation for the oscillation. It is observed that adifferential voltage VCONN, VCONP, controls the frequency of theoscillation. It is further observed that the oscillation includes acombination of a LC oscillator having a feedback control loop comprisingRC components that increase phase noise of the oscillator and thereforethe phase noise could not be decreased because the RC components arenecessary for generation of quadrature output signals.

It is therefore an object of the present invention to present avoltage-controlled oscillator having a relatively wide range ofoscillation and a relatively low phase noise.

In accordance with the invention this is achieved in an oscillator asdescribed in the introductory paragraph characterized in that themodulator means are coupled to amplifier means via an adder forgenerating a quadrature periodical output signal having a frequency in arelative wide range, the frequency being controlled by a control signalprovided to the modulator means. The LC tank circuit has a resonancefrequency that determines an oscillation frequency of the oscillatorthat could be modified either by modifying a capacity in the LC tankcircuit and/or by the control signal provided to the modulator. The LCtank circuit comprises a series connection of an inductor and a resistorcoupled in parallel to a capacitor. When relatively high oscillationfrequencies are generated e.g. GHz the inductor has a relatively highquality factor and a voltage across the resistor is in quadrature withrespect a voltage across the LC tank circuit and therefore theoscillator generates quadrature signals. A feedback signal obtained viathe adder is used for sustaining the oscillation of the oscillator.

In an embodiment of the invention the modulator means comprises a seriescoupling of a buffer and a modulator. The buffer has normally relativelyhigh input impedance and therefore isolates the LC tank circuit from themodulator. Hence, the mixer does not influence quality factor of the LCtank circuit and therefore it's resonance frequency.

In another embodiment of the invention the amplifier means comprise aseries coupling of an another buffer and an amplifier. Whenever it ispossible, a relatively high input impedance buffer is provided forbetter isolating the LC tank circuit from the rest of the circuits.Hence, both the LC tank circuit and the amplifier means work in a morerelaxed conditions in terms of currents and voltages. When relativelyhigh frequencies are generated, the amplifier could be atransconductance amplifier. Transconductance amplifiers amplify avoltage-type input signal and generate a current-type output signal.Current-type signals are very suitable to be used in relatively highfrequency systems a therefore a transconductance amplifier may be usedin generation of relatively high frequency signals.

In an embodiment of the invention the amplifier means is atransconductance amplifier, the modulator means is a Gilbert cellmodulator and the adder is a node. In this current-mode arrangement theadder has the simplest structure i.e. a node and the oscillator may beimplemented with a minimum number of passive components i.e. resistors,capacitors, inductors and active components i.e. transistors,amplifiers. Hence, when integrated in the same chip the circuit occupiesa relatively reduced area.

Because the oscillator has a relatively large tuning range it could beused in many application. In a preferred embodiment the oscillationaccording to the invention is used in a TV tuner having a large tuningrange. Modern tuners normally comprise a phase locked loop, the loophaving a voltage-controlled oscillator that is used for locking on atuned frequency. In order to cover wide frequency range input signals, awide-range voltage-controlled oscillator is necessary and therefore theoscillator according to the present invention may be used.

The above and other features and advantages of the invention will beapparent from the following description of the exemplary embodiments ofthe invention with reference to the accompanying drawings, in which:

FIG. 1 depicts a block diagram of a LC voltage-oscillator according tothe present invention,

FIG. 2 depicts a more detailed block diagram of the LC oscillator,according to the present invention,

FIG. 3 depicts the circuit of the LC tank circuit, according to anembodiment of the present invention,

FIG. 4 depicts the equivalent open-loop circuit of the LC oscillatoraccording to the present invention,

FIG. 5 depicts a transistor level implementation of the inventionaccording to the present invention, and

FIG. 6 depicts a phase locked loop for a TV tuner according to thepresent invention.

FIG. 1 depicts a block diagram of a voltage-controlled LC oscillatoraccording to the present invention. The voltage-controlled oscillatorcomprises a LC tank circuit L, C, R coupled to modulator means 2. Themodulator means 2 are coupled to amplifier means 1 via an adder 3 forgenerating a quadrature periodical output signal having a frequency in arelative wide range, the frequency being controlled by a control signalV_(T) provided to the modulator means 2. The LC tank circuit has aresonance frequency that determines an oscillation frequency of theoscillator that could be modified either by modifying a capacity in theLC tank circuit and/or by the control signal V_(T) provided to themodulator means 2. The LC tank circuit comprises a series connection ofan inductor L and a resistor R coupled in parallel to a capacitor C.When relatively high oscillation frequencies are generated e.g. GHz theinductor L has a relatively high quality factor and a voltage across theresistor V_(R) is in quadrature with respect a voltage across the LCtank circuit V_(O) and therefore the oscillator generates quadraturesignals. A feedback signal obtained via the adder is used for sustainingthe oscillation of the oscillator.

The modulator means 2 may comprise a series coupling of a buffer 20 anda modulator 21, as shown in FIG. 2. The buffer 20 has normallyrelatively high input impedance and therefore isolates the LC tankcircuit from the modulator 21. Hence, the modulator 21 does notinfluence quality factor of the LC tank circuit and therefore itsresonance frequency. The modulator is a Gilbert type modulator. Gilberttype modulators are frequently used in frequency modulation systems.Basically, the Gilbert type modulators output signals are currents andtherefore the modulators are suitable to be used in relatively highfrequency systems wherein current-type signals are easier to be used.Whenever it is possible, a relatively high input impedance buffer 20 isprovided for better isolating the LC tank circuit from the rest of thecircuits. Hence, both the LC tank circuit and the modulator 2 work inmore relaxed conditions in terms of currents and voltages. Whenrelatively high frequencies are generated, the amplifier 11 could be atransconductance amplifier. Transconductance amplifiers amplify avoltage-type input signal and generate a current-type output signal.Current-type signals are very suitable to be used in relatively highfrequency systems a therefore a transconductance amplifier may be usedin high frequency signals generation. In FIG. 2 it is considered thatthe amplifier 11 is a transconductance amplifier and the modulator 21 isa Gilbert modulator. Hence, the adder 3 is a node and the oscillator hasa relatively simple structure.

The LC tank circuit may be modeled as in FIG. 3. In FIG. 3 the feedbackcurrent is denoted as I. The following notations are made: Z₁ = sL + R$Z_{2} = \frac{1}{sC}$wherein s is the complex frequency.

Let's express V_(R) versus V_(O):$V_{O} = {{{I \cdot Z_{1}}//Z_{2}} = {I \cdot \frac{Z_{1}Z_{2}}{Z_{1} + Z_{2}}}}$The two branch currents I₁ and I₂ are given below:$I_{1} = {I \cdot \frac{Z_{2}}{Z_{1} + Z_{2}}}$$I_{2} = {I \cdot \frac{Z_{1}}{Z_{1} + Z_{2}}}$It follows:$V_{R} = {{I_{1} \cdot R} = {{I \cdot \frac{Z_{2}}{Z_{1} + Z_{2}} \cdot R} = {\frac{V_{O}}{Z_{1}} \cdot R}}}$Z₁ can be approximated as Z₁=R+jΩL≈jΩL when the quality factor of theinductor L is much larger than one, meaning that: R<< ω  L${{and}\quad{therefore}},{{Q_{L} = \frac{\omega\quad L}{R}}\operatorname{>>}1}$

-   -   where Q_(L) is the quality factor of the inductor L.        In practice of relatively high frequency systems i.e. GHz, the        quality factor of the inductor L is much larger than one, so        this approximation is valid, resulting that the voltage across        the resistor R is:        $V_{R} = {{\frac{V_{O}}{j\quad\omega\quad L}R} = {{{- j}{\frac{V_{O}}{\omega\quad L} \cdot R}} = {{- j}\frac{V_{O}}{Q_{L}}}}}$

In conclusion the voltage V_(R) is in quadrature with V_(O). This can bemodeled with a stage having a gain equal to 1/Q and a phase shifterhaving a phase shift equal to $- \frac{\pi}{2}$as shown FIG. 4. Using this figure we may calculate the open loop gain,the oscillation frequency and the oscillation condition of theoscillator.Denote$Z_{RLC} = {{\frac{sL}{{s^{2}{LC}} + {sLG}_{p} + 1}\quad{and}\quad G_{p}} = \frac{1}{R_{p}}}$the loss conductance of the LC tank circuit. The total current I_(OUT)of the LC tank circuit is given by$I_{out} = {V_{i} \cdot \left( {G + {\frac{k \cdot V_{T}}{Q} \cdot {\mathbb{e}}^{{- j}\frac{\pi}{2}}}} \right)}$and$V_{O} = {{- V_{i}} \cdot \left( {G + {\frac{k \cdot V_{T}}{Q} \cdot {\mathbb{e}}^{{- j}\frac{\pi}{2}}}} \right) \cdot \frac{sL}{{s^{2}{LC}} + {sLG}_{p} + 1}}$The open-loop gain of the oscillator becomes:${A\quad\beta} = {\frac{V_{O}}{V_{i}} = {{- \left( {G + {\frac{k \cdot V_{T}}{Q} \cdot {\mathbb{e}}^{{- j}\frac{\pi}{2}}}} \right)}\frac{j\quad\omega\quad L}{\left( {1 - {\omega^{2}{LC}}} \right) + {j\quad\omega\quad{LG}_{p}}}}}$or${A\quad\beta} = {- \frac{{\frac{k \cdot V_{T}}{Q}\omega\quad L} + {j\quad\omega\quad{LG}}}{\left( {1 - {\omega^{2}{LC}}} \right) + {j\quad\omega\quad{LG}_{p}}}}$The oscillation condition is Aβ=−1. This means that |Aβ|=1 and φ(Aβ)=π.We may compute now the phase of the open loop transfer φ(Aβ) and thephase condition for sustained oscillations:$\varphi\left( {{A\quad\beta\quad A} = {{\pi + {\arctan\left( \frac{QG}{k \cdot V_{T}} \right)} - {\arctan\left( \frac{\omega\quad{LG}_{p}}{1 - {\omega^{2}{LC}}} \right)}} = \pi}} \right.$This yields the following condition:${\arctan\left( \frac{\frac{QG}{k \cdot V_{T}} - \frac{\omega\quad{LG}_{p}}{1 - {\omega^{2}{LC}}}}{1 + \frac{{{QG} \cdot \omega}\quad{LG}_{p}}{k \cdot {V_{T}\left( {1 - {\omega^{2}{LC}}} \right)}}} \right)} = 0$which is reduced to a second order equation:LCQG  ω² + kV_(T)LG_(p)ω − GQ = 0$\omega_{0} = \frac{\sqrt{4{LCG}^{2}Q^{2}}\left( {\sqrt{1 + \frac{k^{2}V_{T}^{2}{LG}_{p}^{2}}{4{CG}^{2}Q^{2}}} - \frac{{kV}_{T}{LG}_{p}}{2\sqrt{LC}{GQ}}} \right)}{2{LCGQ}}$We may approximate ${\sqrt{1 + X} \approx {1 + \frac{X}{2}}},$when X is much smaller than 1, where$X = \frac{k^{2}V_{T}^{2}{LG}_{p}^{2}}{4{CG}^{2}Q^{2}}$with Q being much larger than one.$\omega_{o} \approx {\frac{1}{\sqrt{LC}}\left\lbrack {1 + \frac{k^{2}V_{T}^{2}{LG}_{p}^{2}}{8{CG}^{2}Q^{2}} - \frac{k\quad V_{T}{LG}_{p}}{2\sqrt{LC}{GQ}}} \right\rbrack}$However $\frac{k^{2}V_{T}^{2}{LG}_{p}^{2}}{8{CG}^{2}Q^{2}}$is negligible compared to $\frac{{kV}_{T}{LG}_{p}}{2\sqrt{LC}{GQ}}$for small tuning voltages. Hence, the oscillation frequency can beapproximated with:$\omega_{o} \approx {\frac{1}{\sqrt{LC}}\left\lbrack {1 - {\frac{{kV}_{T}G_{p}}{2{GQ}}\sqrt{\frac{L}{C}}}} \right\rbrack}$

In the above-expression V_(T) has a bigger effect on the oscillationfrequency to than the capacitance of the LC tank circuit. Therefore,V_(T) may be used for coarse tuning and a varicap diode parallel on theLC tank for fine-tuning. This circuit can be used for large tuning rangeapplications. The series resistors R could have a resistance of 10 ohmswhen the tuning range is one octave i.e. from a frequency f₀ to afrequency 2f₀. The quadrature outputs V_(o) and V_(R) are buffered andamplified in order to produce sufficient power to drive the next stages.

FIG. 5 depicts a transistor level implementation of the inventionaccording to the present invention. A CMOS implementation of the circuitshown in FIG. 5 is also possible and a skilled person in the art couldeasily implement it. Referring to FIG. 5, the LC tank circuit isdifferentially connected to the active part of the circuit. Thetransistors Q7, Q8 are differential buffer 20 and the transistors Q9,Q10 are buffer 10. They are used to decrease the loading effect of theLC tank circuit with the active part, modulator 21 and amplifier 11 ofthe circuit. The transistors Q11, Q12 and the current source I_(T)represent the transconductance amplifier 11. It is connected in apositive feedback manner corresponding to the circuit shown in FIG. 1.The modulator 21 is implemented with transistors Q1, Q2, Q3, Q4, Q5 andQ6. The coarse tuning port C is denoted with VT+ and VT−.

FIG. 6 depicts a phase locked loop for a TV tuner according to thepresent invention. The large tuning range VCO is also employed in PLLcircuits where the quadrature outputs are not always required. However,in zero-IF and low-IF applications the quadrature outputs are mandatory.In low-IF architectures, for cable-TV and terrestrial TV there is a needto cover a large number of bands and that is why the need for largetuning range oscillators or switched oscillators. In the last approachthe number of oscillators reduces to about 4 to 5. Since the coarsetuning in FIG. 6 is differential, the gain of the VCO can be halved andtherefore the differential tuning port of the VCO is less sensitive tocommon-mode noise and substrate noise. The gain of the VCO is relativehigh and therefore the loop should be adapted accordingly. To reducemore the sensitivity of the VCO, digital-tuning techniques may be used.They are based on A/D and D/A converters to decrease the slope of thegain when the VCO is in frequency lock. A possible implementation in aPLL loop with less sensitivity towards perturbations is shown in FIG. 6.The PLL consists of a phase-frequency detector PFD and two paths: aproportional path and an integral path. The integral path applied on thedifferential coarse tuning-port C of the VCO has a pure integrator LPF1in the loop ensuring filtering of the high frequency noise coming fromthe charge-pump and PLL loop. The proportional path has a low-passfilter LPF2 used to drive the FME tuning-port of the VCO. The VCO willbe locked in phase and frequency to the crystal XTAL. The window detectblock WD detects the frequency-lock condition and interrupts the signalcoming from the PFD on the integral part using the logic functions inthe LOGIC block. The octave divider OD has only dividers by two blocksand performs the octave selection function. The quadrature outputs ofthe frequency divider may be used for the I/Q down-conversion for alow-IF, zero-IF receiver. The other blocks of the phase locked loop areknown per se as the frequency divider M, charge pumps CP1, CP2 and thelogic that controls the transfer of the phase-frequency detector PFD tothe Charge pumps CP1, CP2.

It is remarked that the scope of protection of the invention is notrestricted to the embodiments described herein. Neither is the scope ofprotection of the invention restricted by the reference numerals in theclaims. The word ‘comprising’ does not exclude other parts than thosementioned in the claims. The word ‘a(n)’ preceding an element does notexclude a plurality of those elements. Means forming part of theinvention may both be implemented in the form of dedicated hardware orin the form of a programmed purpose processor. The invention resides ineach new feature or combination of features.

1. Voltage controlled oscillator comprising a LC tank circuit coupled tomodulator means and characterized in that the modulator means arecoupled to amplifier means via an adder for generating a quadratureperiodical output signal having a frequency in a relative wide range,the frequency being controlled by a control signal provided to themodulator means.
 2. An oscillator as claimed in claim 1, wherein themodulator means comprises a series coupling of a buffer and a modulator.3. An oscillator as claimed in claim 1, wherein the amplifier meanscomprise a series coupling of an another buffer and an amplifier.
 4. Anoscillator as claimed in claim 3, wherein the amplifier is atransconductance amplifier.
 5. An oscillator as claimed in claim 1,wherein the amplifier means is a transconductance amplifier (, themodulator means is a Gilbert cell modulator and the adder is a node. 6.A phase locked loop comprising an oscillator as as claimed in claim 1for use in a large tuning TV tuner.